One-piece hollow camshafts and process for producing same

ABSTRACT

A method for producing a hollow camshaft comprising i) forming a camshaft preform by providing a hollow metal pipe, expanding the hollow pipe at spaced longitudinal locations by applying pressurized fluid internally to the hollow pipe, axially compressing the hollow pipe during the expanding step to accumulate metal at the spaced longitudinal locations, the expanding and compressing steps forming a camshaft preform having cam preforms of associated predetermined widths in a longitudinal pipe direction; and ii) forming a final camshaft from the camshaft preform by axially compressing the camshaft preform while applying the pressurized fluid to reduce the predetermined widths of the cam preforms while shaping the camshaft preforms into the final camshaft, the final camshaft having cams of relatively smaller widths in the longitudinal direction than the predetermined widths of the cam preforms.

This application claims benefit of international application PCT/DE94/01213, filed Oct. 14, 1994.

BACKGROUND OF THE INVENTION

The invention relates to a hollow camshaft as well as a process forproducing hollow camshafts by employing internal high pressure shapingof hollow pipes.

The invention in particular relates to processes employing pre-shapingin the course of internal high pressure shaping for designing completehollow shafts from pipe sections, wherein pipe sections are hereunderstood to be arbitrary long hollow bodies, i.e. also square pipes,hexagonal pipes or other hollow profiles.

In comparison with solid hollow shafts, hollow camshafts, such asschematically represented in FIG. 1, offer a weight advantage of up to75% over shafts of solid material. In comparison to this, constructedcamshafts such as known from DE-37 04 092, EP 278292, DE-3428372 or EP290758 or EP 303845, only offer a weight advantage of 40% in comparisonwith solid shafts. In addition, constructed camshafts are comparativelyexpensive to produce, because separate cams and hollow pipes must bestocked and then placed together in a die in a suitable manner. Thus,expensive individual components must be kept in stock and combined.

For this reason hollow camshafts, although their constructed shape hasbeen described theoretically and also in patent applications, have notyet been used in motor vehicles up to now.

The reason for this lies in a suitable process technique for producingthese shafts.

Suggestions for producing constructed hollow camshafts are recited, forexample, in DE-19 10 517. These process techniques are, however,completely unusable for producing hollow camshafts from pipe sections.The required degrees of shaping can not be achieved by electromagneticor by electro-hydraulic shaping. Swaging or round kneading as describedin DE-37 36 453 also has not met the goal. This method of producinghollow camshafts fails particularly because of the one-sided massdistribution in the cams. The one-step internal high pressure shaping bymeans of a tool in accordance with U.S. Pat. NO. 2,892,254 also does notmeet the goal. Although axial feeding of material is provided with thismode of operation, filling the die at the cam tips is not achieved and aconsiderable stretching of the walls of the cams results. FIG. 2 showsthe undesired stretching of the walls on a cam which was produced bymeans of the known one-step internal high pressure shaping method.

The internal high pressure method or IHV method is here understood tomean the method which was described for example in the IndustrieanzeigerIndustrial Journal! No. 20 of Mar. 9, 1984, Hohlteile Material-Sparendhergestellt" Hollow Parts Produced in a Material Saving Manner! or in"Materialumformtechik" Material Shaping Technology!, vol. ID/91, pp. 15et seq.: A. Ebbinghaus "Prazisionswerkstucke in Leichtbauweise,hergestellit durch Innenhochdruckumformen" Precision Workpieces inLightweight Construction, Produced by Internal High Pressure Shaping!,or in "Werkstoff und Betrieb" Material and Factory!123 (1990), 3, pp.241 to 243: A. Ebbinghaus "Wirtschaftliches Konstruieren mitinnenhochdruckgeformten Prazisionswerkstucken" Economical Constructionby Means of Precision Workpieces Produced by Internal High PressureShaping!, or "Werkstoff und Betrieb"122, 11, (1989), pp. 933 to 938: A.Ebbinghaus "Gesenkschmiede mit neuer Technologie" Die-Forging With NewTechnology!. To avoid repetition, reference is made in full to theirdisclosure in what follows.

These processes have been used up to now for example for producingflanges, as described in EP-2395052, or for producing constructed hollowcamshafts for fastening cams on a pipe for producing hollow camshafts.

SUMMARY OF THE INVENTION

In contrast to this it is the object of the invention to provide ahollow camshaft which can be economically produced, and a process formanufacturing it.

This object is attained by means of a one-piece camshaft consisting ofmetal or plastic which can be cold-shaped and heat-treated and has aflow of fibers which is parallel to the outer contours of the camshaft.

Carbon-containing steel, which can be sufficiently stretched, but canalso be heat-treated for hardening, can preferably be used as thematerial for this.

Suitable materials can be: 19MnB4, 16MnCr5, CK45, 19CrKo44, 15Cr3,C25-steel, X3NiCoMoTilB 9 5, β-C-titanium alloys and those showingsimilar properties.

The invention furthermore relates to a method for producing hollowshafts, employing the internal high pressure shaping of hollow pipes,distinguished by the following steps:

Placing a pipe outlet element and filling it with fluid;

Sealing at least the pipe section to be widened;

Applying an internal high pressure suitable for widening the pipesection to produce an intermediate product preform, wherein, while theinternal high pressure is applied, the hollow pipe is compressed againsta movable die in the direction of its long axis in such a way thataccumulations of material occur at approximately those places where itis intended to create cams; and

Shaping the intermediate product preform into a shape corresponding to afinal camshaft shape by means of the internal high pressure shapingmethod, so that cams are shaped at the desired locations; if necessary,finishing the cams by heat-treatment or the like.

Other advantages will be appreciated from the following disclosure andclaims.

Thus, proceeding beyond the known internal high pressure shaping method(see "Tagungsband des 14. umformtechnischen Kolloquiums" Minutes of the14th Colloquium on Shaping Technology! in Hannover 1993, for thedisclosure of which reference is made in full), in accordance with theinvention a special two-step shaping method is provided, by means ofwhich hollow camshafts can be economically produced. In the process,exactness of shape and low wall stretching at the cam tips is achievedin an appropriate internal high pressure shaping method.

In a surprising manner it is possible to produce camshafts havingessentially constant wall thickness.

The invention will be explained in detail below, making use of theattached drawings and the subsequent description.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of a camshaft in accordance withthe invention, produced in two shaping steps;

FIG. 2A is a schematic representation of a camshaft produced in a singleshaping step in accordance with the prior art;

FIG. 2B is a section along the line 2B--2B of FIG. 2A;

FIG. 3A is a section through a cam preform made in accordance with theinvention;

FIG. 3B is a section through the final form of the cam created from thepreform of FIG. 3A;

FIG. 4A is a cross-sectional view of a cam preform after a first shapingstep;

FIG. 4B is a partial longitudinal sectional view of the cam preformshown in FIG. 4A;

FIG. 5A is a cross-sectional view of a second cam preform after a firstshaping step;

FIG. 5B is partial longitudinal sectional view of the cam perform isshown in FIG. 5A;

FIG. 6A is a cross-sectional view of a third cam preform after a firstshaping step;

FIG. 6B is a partial longitudinal sectional view of the cam preform inshown in FIG. 6A;

FIG. 7A is an enlarged cross-sectional view of a portion of the finalcam shown in FIG. 1, and produced from the preforms shown in FIGS. 4 to6;

FIG. 7B is a partial longitudinal sectional view of the final cam shownin FIG. 7A; and

FIG. 8 is partial longitudinal sectional view of a final cam shaftmanufactured in accordance with the present invention.

DETAILED DESCRIPTION OF THE INVENTION

A camshaft in accordance with the invention, generally indicated at 10in FIG. 1, is distinguished in that, in contrast to conventionallyconstructed hollow camshafts, it does not have attached cams, but isproduced from a single original pipe without any further individualelements in several shaping steps. As a result, there is an extremelyadvantageous fiber flow extending parallel to the outer contours of theshaft, which results in great sturdiness of the shaft and thus inreduced weight. If desired, it is possible at the same time to set anessentially constant wall thickness of 2 mm to 5 mm, for example, whichhas a tolerance of only approximately 0.5 mm.

As shown in FIG. 1, a hollow tube has been expanded at spacedlongitudinal locations 62, 64 to form the hollow camshaft in accordancewith the invention.

No problems of a possible detachment of the cams from the central pipeoccur. It is important that a material is used which has sufficientcold-forming capability while at the same time has hardness or thecapability of being hardened in order to create wear-resistant cams.

This can be, for example, suitable carbon-containing steel which can beheat-treated but also is sufficiently stretchable for tolerating suchextensive shaping without tearing. Depending on the intended use of theshaft in stationary engines, aircraft engines, ship's engines or landvehicles, it is of course possible to employ other light and/or sturdymaterials, such as aluminum alloys, titanium alloys and other materialsfamiliar to one skilled in the art, such as 19MNB4, 16MnCr5, CK45,19CrKo44, 15Cr3, C25-steel, X3NiCoMoTilB 9 5, β-C-titanium alloys andthose showing similar properties. Depending on the intended use it ispossible to employ an expensive, light material--this is suggested forexample for aircraft engine construction--or a heavier material ifweight is not an essential consideration, such as for ship's engines.

The method in accordance with the invention preferably provides at leasta two-step shaping process consisting of preform and finished form,wherein particular importance is assigned to the preform in the firstshaping step. So that exactness of shape and little wall stretching areassured to the finished form in the second shaping step, it is necessaryto provide an advantageous mass distribution of the cam wall in thepreform. In each case the mass distribution can only be achieved if thematerial is being axially pushed longitudinally inwards on oppositesides of each cam by a movable die. The axial feeding at each cam isperformed by means of tool technology known per se.

FIG. 2B shows a section through a hollow camshaft formed by means of asingle shaping step in accordance with the prior art, and it can be seenparticularly from the cross section of the cam in FIG. 2B thatstretching of the metal in the one-step method is too high and thatconsiderable differences in wall thickness occur, namely a wallthickness of approximately 1.0 mm at the cam tip 12 and of 3.0 mm on thepipe wall 14 opposite the cam tip. Because of this, sturdiness is notassured.

In contrast to this, a cam as produced in accordance with the method ofthe present invention is first formed into a preform 16, as showngenerally in FIG. 3A. It can be appreciated that the cam preformillustrated in FIG. 3A has relatively rounded tip ends 20 in comparisonwith the relatively pointed cam tip ends 22 in the final camshaft, asshown in FIG. 3B. Because initially a cam is produced into the preform16 which is wider, preferably approximately 10 to 40% wider andparticularly preferred approximately 20 to 30% wider than the finishedcamshaft form 10, as shown generally in FIG. 3B, and is also more roundthan the finished form 10, it is possible to provide in the preform 16 asufficient accumulation of material at the critical cam location 20where maximum material stretching takes place. This accumulation ofmaterial can then in a second shaping step be shaped into the finishedcam form 10, for example by means of an internal high pressure shapingmethod. However, if desired, other shaping methods, such as swaging, canalso be employed. It can be seen that for the finished camshaft 10, thedesired accumulation of material under the cam 22 can be achieved bymeans of two shaping steps.

In the embodiment shown in FIG. 4A, for example, the cam preform 26 hasround cam tips R1 in order to prevent unnecessary material stretching.The cam preform width B1, as shown in FIG. 4B, is wider by 10 to 40%,preferably 20 to 30%, than the finished form width B2, as shown in FIG.7A. In the course of shaping the finished form, the round and readypreform 26 is axially compressed in the second shaping step, resultingin the finished camshaft 10 illustrated in FIGS. 7A and 7B. Thiscompressing process prevents the excessive stretching of the pipe walland forces a high degree of shape accuracy, which makes late finishingunnecessary.

FIGS. 5A and 5B show another possible preform variant indicated at 32,for the first shaping step, with a directedly increased circumference U1of the cams in the preform which is approximately 5 to 20% greater,preferably 10% greater than the finished form. In this case the materialflows in the circumferential direction U2 into the cam tip in the courseof producing the finished form. In this connection reference is made toFIGS. 6A and 6B showing a partially collapsed cam shape which then isshaped into a cam in a subsequent shaping step.

The finished cam shape is then represented in FIGS. 7A and 7B and can beproduced from every one of the preforms of FIGS. 4A, 5A and 6A. In oneembodiment--to which the invention is not limited in any way--theexterior cam height is 50 mm from the cam tip 40 to the cam base 42; thepipe section has a diameter of 30 mm and a wall thickness ofapproximately 3 mm, the cam tip 40 has a wall thickness of 2 to 3 mm andthe cam base 42 has a wall thickness of up to 5 mm, since the greatestaccumulation of material takes place there. The cam preforms of FIGS.4A, 5A, and 6A and finished forms represented in these figures (e.g.,FIG. 7A and 7B) are of course not limiting. If required, it is alsopossible to form bearing rings and the like on the camshaft 10 inaccordance with this principle.

FIG. 8 is a cross-sectional view of the camshaft 10 of the invention. Inaccordance with the present invention, the camshaft 10 has fibers thatextend parallel with respect to the walls of the camshaft.

Finally, the possibility of a combination of internal high pressureshaping-methods and other shaping methods should be mentioned. Asuitable preform can be produced, for example, by means of internal highpressure shaping and can be subsequently finished by swaging or othersuitable methods familiar to one skilled in the art.

The shaft can be further processed in accordance with the IHV method ina known manner. For example, the shaft can be hardened. Useful for thisare the known processes, such as finishing of the shaft by means of oneor several finishing methods selected from the group comprisingnitrocarbonizing, plasma-nitriding, treating with boron, laserhardening, hardening without carbonizing, induction hardening, flamehardening, electron-beam hardening and case hardening.

It is pointed out that the shafts in accordance with the invention canbe produced by means of the IHV method with the most varied cam angles,wall thickness, wall thickness ratios and are in no way limited to anyone of the described embodiments.

We claim:
 1. A method for producing a hollow camshaft comprising:i)forming a camshaft preform having cam preforms of respective widths in alongitudinal pipe direction by:providing a hollow metal pipe; expandinga diameter of said hollow pipe at spaced longitudinal locations byapplying pressurized fluid internally to said hollow pipe; axiallycompressing said hollow pipe during said expanding step to accumulatemetal at said spaced longitudinal locations; and ii) forming said hollowcamshaft from said camshaft preform by:axially compressing said camshaftpreform while applying said pressurized fluid to reduce thepredetermined widths of said cam preforms while shaping said camshaftpreform into the hollow camshaft, said hollow camshaft having cams ofsmaller widths in the longitudinal direction than the widths of said campreforms.
 2. The method according to claim 1, wherein said predeterminedwidths of said cam preforms are between 10% to 40% greater than thewidths of the cams of the final camshaft.
 3. The method according toclaim 2, wherein said cam preforms have rounded tip ends, and saidmethod further comprises a step of forming cam tip ends which are morepointed in comparison to said rounded tip ends.
 4. The method accordingto claim 3, wherein said final camshaft is hardened using a methodselected from the group comprising: nitrocarbonizing, plasma-nitriding,boron treatment, laser hardening, induction hardening, heat treatment,electron beam hardening and case hardening.
 5. The method according toclaim 1, wherein said predetermined widths of said cam preforms arebetween 20% to 30% greater than the widths of cams of said finalcamshaft.
 6. The method according to claim 5, wherein said cam preformshave rounded tip ends, and said method further comprises a step offorming cam tip ends which are more pointed in comparison to saidrounded tip ends.
 7. The method according to claim 1, further comprisingthe step of axially pushing on both sides of each of said cam preforms.8. A method for producing a hollow camshaft comprising:i) forming acamshaft preform having a plurality of cam preforms of respectivepredetermined circumferences by:providing a hollow metal pipe; applyingpressurized fluid internally to said hollow metal pipe to expand adiameter of said hollow pipe at spaced longitudinal locations; axiallycompressing said hollow pipe while applying said pressurized fluid toaccumulate metal at said spaced longitudinal locations; and ii) formingsaid hollow camshaft from said camshaft preform by: axially compressingsaid camshaft preform and applying pressurized fluid internally to saidcamshaft preform so as to shape said camshaft preform into said hollowcamshaft, reducing the circumference of said cam preforms during saidaxially compressing step so as to form said hollow camshaft with cams ofa final circumference less than said predetermined circumference.
 9. Themethod according to claim 8, wherein said predetermined circumference isbetween 5% and 20% greater than said final circumference.
 10. The methodaccording to claim 9, wherein said predetermined circumference is 10%greater than said final circumference.
 11. The method according to claim10, wherein said cam performs have rounded tip, ends and said methodfurther comprises a step of forming cam tips ends which are more pointedin comparison to said rounded tip ends.
 12. The method according toclaim 8, wherein said cam preforms have rounded tip ends, and saidmethod further comprises a step of forming cam tip ends which are morepointed in comparison to said rounded tip ends.
 13. The method accordingto claim 8, wherein said final camshaft is hardened using a methodselected from the group comprising: nitrocarbonizing, plasma-nitriding,boron treatment, laser hardening, induction hardening, heat treatment,electron beam hardening and case hardening.
 14. The method according toclaim 8, further comprising the step of axially pushing on both sides ofeach of said cam preforms.